Numerical Study of Sandbar Migration under Wave-Undertow Interaction

Reliable simulation of onshore-offshore sandbar migration under various wave and current conditions has remained a challenging task over the last three decades because wave-undertow interaction in the surf zone has been neglected in the existing numerical models. This paper presents the development of an improved sandbar migration model using a phase- and depth-resolving modeling approach. This new model includes interactions between waves and undertow and an empirical time-dependent turbulent eddy viscosity formulation that accounts for the phase dependency of turbulence on flow velocity and acceleration. The authors demonstrate through extensive model-data comparisons that these enhancements resulted in significant improvements in the predictive capability of the cross-shore sandbar migration beneath moderate and energetic waves. The comparison showed wave-undertow interaction playing a crucial role in cross-shore sediment transport. Waves increased the undertow-induced suspended-load flux during offshore sandbar migration, and a weak undertow suppressed the wave-induced onshore bed-load transport during onshore sandbar migration. The proposed empirical time-dependent turbulent eddy viscosity significantly improved the prediction of onshore-directed bed-load transport during onshore sandbar migration.